1. Field of the Invention
The present invention relates to a coding method of a motion vector, and more particularly, to a method for coding a motion vector using a 2-dimensional minimum bitrate predicting technique.
2. Description of the Background Art
In general, a motion vector is coded by a lossless coding in a mobile image compression. In such a case, a differential pulse code modulation (DPCM) technique is mostly used which is divided by a portion for computing a prediction value of a motion vector to be currently coded and a portion for coding a prediction error between the computed prediction value and the current motion vector.
Accordingly, a mobile image compression performance depends on how successfully the current motion vector is predicted by using a neighboring motion vector which has been already transmitted.
Conventional motion vector coding method roughly includes a Median predicting technique and one-dimensional minimum bitrate predicting technique.
The Median predicting technique is mostly used for an H.263 and an MPEG-4, in which a median value of previously transmitted neighboring motion vectors is computed as a prediction value of a motion vector to be currently coded and only a prediction error between the computed prediction value and the current motion vector to be coded is coded and transmitted.
In detail, as shown in FIG. 1, on the assumption that a motion vector to be currently coded is MV and neighboring motion vectors are MV1, MV2 and MV3, a motion vector coding method using the conventional median predicting technique can be implemented in the form as shown in FIG. 2.
First, a predictor 10 obtains a median value of the previously transmitted neighboring motion vectors MV1, MV2 and MV3 and computes a prediction value (PMV) for the motion vector MV to be currently coded.PMVX=Median (MV1x, M2x, M3x)PMVY=Median (MV1y, M2y, M3y)
After the prediction value (PMV) is computed, a subtracter 12 subtracts the prediction value (PMV) outputted from the predictor 10 from a motion vector (MV) to be currently coded in order to compute a prediction error (MVD).MVDx=MVx−PMVxMVDy=MVy−PMVy
The prediction value (PMV) and the prediction error (MVD) are processed by X and Y factors.
An encoder 14 performs a variable length coding on the prediction error (MVD) outputted from the subtracter 12 and transmits it to a destination.
Meanwhile, the one-dimensional minimum bitrate predicting technique is a method of checking to see which one of the neighboring motion vectors should be used as a prediction value to generate the smallest (the minimum bitrate) prediction error (MVD), and then the detected minimum bitrate prediction error (MVDmbp) and MODE information indicative of a corresponding motion vector are transmitted together.
First, the minimum bitrate predicting technique computes a prediction error (MVD) generated when the neighboring motion vectors (MV1, MV2 and MV3) are respectively used as a prediction value, for a factor ‘X’. As shown in FIG. 3A, assuming that MV=1, MV1=0, MV2=3 and MV3=4.5, the prediction errors (MVDx) of the motion vectors (MV1, MV2 and MV3) for the factor ‘X’ are respectively +1.0, −2.0 and −3.5.
Once the prediction errors (MVDx) are computed, the encoder 14 selects an MVD value (MVDmbp) which generates the minimum bitrate among the computed prediction errors and codes the MVD value first.
Thus, on the assumption that the bitrate is in proportion to an absolute value of the prediction error (MVD), since the motion vector (MV1=0) generates the smallest bitrate, the encoder 14 codes the ‘MVDxmbp=+1.0’ with a variable length code and transmits it.
If the minimum bit prediction error (MVDxmbp=+1.0) is transmitted, the encoder 14 is to transmit mode information (MODEX) indicating that the MV1 has been used as a prediction value. For this purpose, as shown in FIG. 3B, the encoder 14 defines a candidate motion vector that the decoder can obtain only with the minimum prediction error (MVDxmbp=+1.0) and the neighboring motion vectors (MV1, MV2 and MV3).
In other words, the encoder 14 searches out candidate motion vectors (MVC1, MVC2 and MVC3) by using the minimum prediction error (MVDxmbp=+1.0) and the previously transmitted neighboring motion vectors (MV1, MV2 and MV3).
The MVC1 is a motion vector candidate value obtained by using MVDxmbp when the MV1 has been used as a prediction value, and MVC2 is a motion vector candidate value obtained by using the MVDxmbp when the MV2 has been used as a prediction value. Also, the MVC3 is a motion vector candidate value obtained by using the MVDxmbp when the MV3 has been used as a prediction value.
Among them, however, MVC2 is failed to be a real candidate value. The reason for this is that if the MVC2 is a motion vector to be substantially transmitted. −0,5, rather than +1,0, as a minimum prediction error (MVDxmbp) should have been transmitted by having MV2 as a prediction value. Thus, since MVC1 and MVC3 are the actually effective candidate motion vectors, the encoder generates mode information (MODEx) as information of 1 bit (‘0’ or ‘1’) and transmits it.
After the minimum prediction error (MVDxmbp) and the mode information (MODEx) for the factor ‘X’ are completely transmitted, the encoder 14 performs the same process to obtain a minimum prediction error (MVDymbp) and mode information (MODEy) for a factor ‘Y’ and transmits them. FIG. 4 illustrates the structure of the bit stream which is transmitted at this time.
However, the conventional motion vector coding method using the Median predicting technique and the one-dimensional minimum bitrate predicting technique has a problem that since the factors ‘X’ and ‘Y’ of the motion vector are independently processed, causing a disturbance to a real time compression coding and an increase in the amount of the transmitted data,
Especially, in case of the motion vector coding method using the one-dimensional minimum bitrate predicting technique, since the factors ‘X’ and ‘Y’ exist also in the mode information as well as in the prediction error information, transmission burden of the mode information is much increased.
In addition, the prediction error (MVD) generated according to the minimum bitrate predicting technique is more converged to (0,0) compared to the prediction error (MVD) obtained by the Median predicting technique. But the conventional motion vector coding method fails to provide a solution which may allow to effectively use a distribution characteristic change of the prediction error (MVD) generated due to the prediction technique.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.